1. Field of the Invention
The invention relates to apparatus for removing a liquid from a mixture of liquids, in particular separating immiscible liquid of differing densities for example oil from a mixture of oil and water, comprising a collecting tank for the mixture of liquids and an overflow, a drain box with a weir arranged in the collecting tank, the liquid to be separated passing over the weir into the drain box. In addition the invention relates to a method for operation of this apparatus.
2. Description of the Prior Art
Such separation installations are known in many branches of industry. Separation installations for removing oil from water/oil mixtures are used in sea-going and inland waterways vessels to treat the water found in the bilges, and in industry for the regeneration of oil from an oil/water mixture.
An example of the latter application can be found in the steel industry, where in the cold rolling process oil, which is provided during rolling to limit friction losses, is mixed with cooling water which is sprayed onto the rolls. The cooling water sprayed onto the rolls and the oil both form part of a single circulation system. That means that the cooling water is collected in a collecting tank to cool down after being sprayed on the rolls, after which following cleaning it is used again. The high cost of the oil used together with the large consumption (for example 4900 liters per day) makes the recovery of this oil economically very desirable.
In one known technique this recovery is carried out in two stages. The liquid running over the weir of separating apparatus as described above is subjected to a centrifuge process in two stages. The first centrifuge operation serves to separate from the oil the water that has accidently run over the weir. The quantity of water which accidentally passes over the weir is particularly great with respect to the quantity of oil; it amounts for example to approximately eight times the quantity of oil. The second centrifuge stage serves to remove other impurities from the oil. The centrifuge capacity required is largely determined by the quantity of liquid to be processed in the first process stage. In practice, this first process stage accounts for 80% of the capacity. In view of the costs involved in centrifuging, and in the further finishing of the separated water, one object of the present invention is to reduce the quantity of water relative to the quantity of oil passing over the overflow. As a result the centrifuge capacity required, in particular in the first process stage, will be reduced and in addition the finishing costs of the separated water will fall.
A second object of the invention relates to the reduction of the thickness of the layer of oil floating in the collecting tank. The aim is that the water removed from the collecting tank will contain less oil than in the known apparatus. The water from the collecting tank is in practice used as cooling water during rolling. Too much oil in the cooling water results in a reduction in the cooling effect because the used oil has the property of forming a skin around the water droplets, which makes it more difficult for heat to be transferred from the roll to the water.
FR-A-No. 2 426 288 describes apparatus for separating liquids in which a drain box floats in the liquid mixture in a container. The top edge i.e. the weir, of the drain box is movable relative to the rest of the drain box. A float on the liquid level in the drain box controls the position of the top edge. Thus when the drain box becomes fuller and sinks lower in the liquid mixture, the weir level rises relative to the bottom of the drain box, with the aim of preventing excessive flow of liquid into the drain box. This apparatus does not permit precise control of the flow of liquid into the drain box.